DESCRIPTION (Applicant's Abstract): The long term goal of this project is to define the consequences of the interaction of cocaine with presynaptic dopaminergic systems. Many of the behavioral effects of cocaine are attributed to its ability to block the reuptake of dopamine in mesolimbic neurons. Consequently, increased extracellular dopamine may enhance postsynaptic transmission and/or modulate presynaptic dopamine receptors known to inhibit neurotransmitter synthesis as well as further release. The specific aim of this application is to dissect this system by focussing on presynaptic events associated with cocaine's purported modulation of dopamine autoreceptors. Towards this end, model neuronal systems have been engineered by transfecting immortalized mesencephalic dopamine producing cell lines with cloned D2 and D3 receptors. To test the hypothesis that the D2-like receptors subserve different autoreceptor roles, the transfected cell lines are being systematically tested for receptor mediated effects on synthesis and release. Previously we have shown that agonist stimulation of D2 and D3 receptors leads to subtype specific reductions in dopamine release and synthesis. These data imply specific roles for each receptor and suggest the effects of cocaine may vary depending upon receptor subtype. Studies outlined in this proposal will 1 ) test the hypothesis that the differential autoreceptor effects of D2 and D3 receptors are due to distinct coupling mechanisms; 2) determine the mechanistic basis for D3 desensitization; 3) test the hypothesis that in primary cultures of neurons the dopamine D2 receptor subtype can serve all these functions: modulation of firing rate, dopamine synthesis and dopamine release; and 4) test the acute and chronic effects of cocaine on autoreceptor function in transfected cell lines and primary dopaminergic cultures. Taken together these studies will significantly extend our ongoing efforts to understand the complex receptor/presynaptic effector interactions involved in the reinforcing and behavioral sensitization effects of cocaine.